This invention relates to computed tomography and is particularly concerned with determining the CT numbers of zones of interest in an image displayed on a cathode ray tube which zones lie in the so-called level or center of the gray scale window.
In computed tomography, CT numbers are proportional to the X-ray attenuation by small volume elements in the body being scanned with an X-ray beam. A feature of the invention is to cause zones in the computer reconstructed image corresponding with a small range of CT numbers to blink in the context of the entire image or picture being displayed such that the CT numbers of the zones may be identified.
In computed tomography, X-ray transmission characteristics are measured along a plurality of paths through a body undergoing examination. Signals corresponding with the intensity of the X-rays in the various paths are processed by a computer which develops a matrix of CT numbers that govern the intensity or brightness of picture elements (pixels) which make up the displayed image. An illustrative method of image reconstruction with computed tomography is described in U.S. Pat. No. 3,778,614 to Hounsfield, which is incorporated by reference as background material in this disclosure.
A variety of algorithms had been developed for controlling a computer to convert X-ray attenuation data of each volume element in a layer of the body being examined with a scanned X-ray beam to a matrix of CT numbers that are usable to control a cathode ray display tube (CRT). X-ray attenuation by the volume elements is expressed as a CT number which is defined as follows: ##EQU1## WHERE U.sub.W IS THE X-ray absorption coefficient of water and u.sub.t is the X-ray absorption coefficient of tissue.
The value of K in this equation depends on the number of gray scale gradations into which the full absorption range is divided. For a system using 1024 gray scale gradations, K = 500. Systems using 256, 1024 and 2048 gray scale gradations or CT numbers have been used and higher ranges could be used.
The computed CT numbers are stored in a memory and, with a known type of display controller, their voltage analogs may be used as z signals to modulate the intensity of the beam in a raster scanned cathode ray tube for producing a visible image.
The detectors which are used for sensing the large amount of X-ray data and for providing intensity signals to the computer are extremely sensitive to X-rays. Using data provided by the detectors, the computer algorithm operates to produce a range of CT numbers with greater X-ray attenuation resolution or gray scale steps than can be distinguished by the human eye in a cathode ray tube display. For example, present tomographic image reconstruction methods are capable of quantitizing X-ray attenuation measurements into 1024 or more separate levels as indicated above. Cathode ray tubes, on the other hand, are typically incapable of displaying more than approximately 64 visually discernible shades or gray scale levels. Hence, it has been the practice to select from a large range of CT values, such as 1024, a limited range called a window and to display picture elements having CT values within the window over the gray scale capability of the CRT in which case CT values above the upper window limit are white and those below the lower window limit are black.
Significant medical information such as the presence or absence of tumors in soft tissue is often represented by minimal CT number differences. Those practicing the computed tomography art foresee the possibility of correlating the CT numbers of zones in the displayed image with the nature of the tissue involved and with the presence or absence of pathology in the tissue. For example, a benign cyst might fall within one small range of CT numbers and a malignant tumor or healthy tissue may fall within another range. If the relationship between CT numbers and the condition of tissue can be accurately correlated by further experience, a substantial diagnostic aid will be achieved.
Some computed tomography systems now in use provide for identification of CT numbers corresponding with particular zones in a displayed image. CT number identification in prior apparatus involves displaying the picture within a selected gray scale range or window. A level is selected which corresponds with the center of the gray scale within the window. A window is also selected which, by way of illustration and not limitation, could have an upper CT number limit of +100, corresponding with white, and a lower limit of -100, corresponding with black, in which case the gray scale would be expanded over a range of CT numbers equaling 200 and the center or level setting would be 0 in this example.
The prior procedure for determining the CT number of a zone of interest in a displayed image was to narrow the window down to a range of two CT values so that there would be one CT value at the level setting and one below it. Thus, the gray scale was compressed to two gradations, black and white, and all pixels in the displayed image on the screen of the display tube appeared either white or black. The level control could then be adjusted to a condition at which a particular zone of interest within the display would be caused to change from black to white. The level setting which caused this change could then be interpreted as the CT value of the zone of interest. If the CT number of any other zone was required, the level or center of the gray scale was set to correspond with it and the window was reduced or converged to this particular level. Again, any zone in the displayed picture with a higher CT number than that which corresponded with the level setting would appear white and any below the level would appear black so that the CT numbers corresponding with the level could be identified.
A disadvantage of the prior practice just described is that the zones whose CT numbers were sought to be identified could not be visualized in the context of the displayed image since when the window was reduced to the range of plus and minus one value relative to the set level, all picture information outside of that range was obliterated. That is, only island-like zones having CT numbers near the level setting were visible on the screen of the cathode ray tube. Another disadvantage was that the window had to be reset and the displayed picture reestablished before the radiologist could continue his study of the entire image.
In accordance with the present invention, the CT number of all zones in the visible displayed image having corresponding brightness or grayness may be determined within the context of the image on the screen of the cathode ray tube, that is, without obliterating the image or changing the window settings of the gray scale. This involves setting the center, called the level, of the gray scale window to correspond with the intensity or grayness of the zone of interest in the image. The window limits are selected previously. When CT number identification of any selected zone is desired, the operator simply presses an identification command button which causes all picture zones at the selected level to blink or turn white and then go back to normal gray level. When the selected zones blink to white, other zones comprising the image continue to be displayed in their normal gray scale and, of course, between blinks even the selected zones are displayed in their normal gray scale. The CT number is easily determined by simply reading the level setting which corresponds with the zone starting to blink.
Accordingly, a principal object of this invention is to provide a method and means for causing picture elements, in a cathode ray tube display of a computer reconstructed image, that correspond with the center of the gray scale or level, to blink for enabling rapid identification of the CT numbers of those pixels.
Another important object and feature of the invention is to provide for identifying the CT number of selected image zones in the context of the image itself without obliterating the remainder of the image and without the need for making any adjustments or settings to restore the displayed picture after using the blink mode.